Display devices are known in which a liquid crystal material is placed between two transparent plates having on the inner surfaces thereof mosaic patterns or numerical segments which are transparent and conductive and which can be connected to an external source of voltage. The inner surfaces of the electrodes are rubbed with gauze, cotton, absorbent cotton, etc., each of the electrode inner surfaces being rubbed in a single direction. In joining the two transparent plates to form a liquid crystal display cell, the plates are oriented so that the rubbing directions on the opposed plates are at right angles to each other.
When suitable liquid crystal material is placed between such plates, the liquid crystal molecules adjacent each of the plates align themselves with the rubbing directions of the adjacent plate. The molecules lying between the plates then orient themselves so as to form a quarter-turn helix between the plates. In the absence of an electric field, such a liquid crystal material will rotate the plane of polarized light through an angle of 90.degree..
When a liquid crystal cell constructed as described is placed between two polarizing filters, the axes of which are at right angles to each other, then incident light will be transmitted through the cell. However, if an electric field of sufficient strength is imposed across the cell through use of an external source of voltage in combination with the transparent electrodes on the inner surfaces of the plates, then the optical activity of the liquid crystal material can be decreased to zero, due to the fact that the molecules will tend to align themselves with the electric field. As a result, incident light will be blocked from passage through the cell.
In order for liquid crystal material to function as described, the composition must have a high positive dielectric anisotropy. When an electric field is imposed upon such a material, the material is transformed from one having double refraction to an isotropic liquid. This property is valuable since a cell containing such a liquid can be used as an optical shutter and, moreover, the shutter functions only in those regions of the cell across which the electric field is applied.
Nematic liquid crystals having positive dielectric anisotropy have a structure such that the dipole moment of the liquid crystal molecule lies along the direction of the major axis thereof. It is for this reason that the molecule, in the presence of a sufficiently strong electric field, orients itself so that the major axis of the molecule is aligned with the electric field.
Most of the nematic liquid crystals of high dielectric anisotropy known up to the present time have been of the Schiff-type or of the ester type as shown by the following structural formulae: ##SPC1##
These materials have relatively poor chemical stability and are degraded easily by light and by moisture. The effect of moisture is particularly severe since when an electric field is applied, conduction results with consequent oxidation and reduction as well as hydrolysis of the material.
In attempting to find liquid crystal materials suitable for display devices of the type under consideration, the following relationship between the dielectric anisotropy .DELTA..epsilon. and the operative threshold voltage is pertinent: ##EQU1## where .DELTA..epsilon. = .epsilon..sub..parallel. - .epsilon..sub..vertline.,
.epsilon..sub..vertline. and .epsilon..sub..parallel. being the dielectric constants of the molecule in the directions perpendicular and parallel to the molecular axes respectively. The value of k is such that if a nematic composition has a .DELTA..epsilon. as large as 10 - 12, then a display device can be driven by a voltage as low as 1.5.